1. Field of the Invention
The present invention relates to a Planar Inverted F Antenna (PIFA) and in particular to a method of designing a single band PIFA as an encapsulated module with a localized ground plane and multiple external lead contacts for easy integration to the chassis of a radio communication device.
2. Description of the Related Art
With the rapid progress in wireless communication technology and the ever-increasing emphasis for its expansion, wireless modems on laptop computers and other handheld radio devices will be a common feature. The technology using a short-range radio link to connect devices such as cellular handsets, laptop computers and other handheld devices has already been demonstrated [Wireless Design On-line Newsletter, Vol. 3, Issue 5, Nov. 22, 1999]. The ISM band (2.4-2.5 GHz) is the allocated frequency band for such applications. The performance of the antenna placed on devices like a cellular handset or a laptop computer is one of the critical parameters for the satisfactory operation of such a radio link. Therefore the performance characteristics of the antenna located on communication devices assumes significant importance in the evolving technology of wireless modems.
Recently, in the cellular communication industry, there has been an increasing emphasis on internal antennas instead of conventional external wire antennas. The concept of an internal antenna stems from the avoidance of a protruding external radiating element by the integration of the antenna into the device itself. Internal antennas have several advantageous features such as being less prone to external damage, a reduction in overall size of the handset with optimization, and easy portability. In most internal antenna designs, the printed circuit board of the communication device serves as the ground plane of the internal antenna. Among the various choices for internal antennas, a PIFA appears to have great promise. The PIFA is characterized by many distinguishing properties such as relative light weight, ease of adaptation and integration into the device chassis, moderate range of bandwidth, Omni-directional radiation patterns in orthogonal principal planes for vertical polarization, versatility for optimization, and multiple potential approaches for size reduction. The PIFA also finds useful applications in diversity schemes. Its sensitivity to both vertical and horizontal polarization is of immense practical importance in mobile cellular/RF data communication applications because of absence of the fixed antenna orientation as well as the multi-path propagation conditions. All these features render the PIFA to be a good choice as an internal antenna for mobile cellular/RF data communication applications.
A conventional prior art single band PIFA assembly 100 is illustrated in FIGS. 9 and 10. The PIFA 100 shown in FIG. 9 and 10 consists of a radiating element 101, a ground plane 102, a power feed hole 103 is located corresponding to the radiating element 101, a connector feed pin 104, and a conductive post or pin 105. The connector feed pin 104 serves as a feed path for radio frequency (RF) power to the radiating element 101. The connector feed pin 104 is inserted through the feed hole 103 from the bottom surface of the ground plane 102. The connector feed pin 104 is electrically insulated from the ground plane 102 where the pin passes through the hole in the ground plane 102. The connector feed pin 104 is electrically connected to the radiating element 101 at 106 with solder. The body of the feed connector 107 is electrically connected to the ground plane 102 at 108 with solder. The connector feed pin 104 is electrically insulated from the body of the feed connector 107. A through hole 109 is located corresponding to the radiating element 101, and a conductive post or pin 110 is inserted through the hole 109. The conductive post 110 serves as a short circuit between the radiating element 101 and the ground plane 102, The conductive post 110 is electrically connected to the radiating element 101 at 111 with solder. The conductive post 110 is also electrically connected to the ground plane 102 at 112 with solder. The resonant frequency of the PIFA 100 is determined by the length (L) and width (W) of the radiating element 101 and is slightly affected by the locations of the feed pin 104 and the conductive post or shorting pin 110. The impedance match of the PIFA 100 is achieved by adjusting the diameter of the connector feed pin 104, by adjusting the diameter of the conductive shorting post 110, and by adjusting the separation distance between the connector feed pin 104 and the conductive shorting post 110.
In the prior art techniques of PIFA design (Murch R. D. et al, U.S. Pat. No. 5,764,190; Korisch I. A., U.S. Pat. No. 5,926,139) the center conductor of the coaxial cable from the RF source is directly connected to the radiating element of the PIFA at the feed point. Further, in all these designs, the feed point of the PIFA is always drawn away from the shorted edge of the radiating element and is located within the central surface of the radiating element. Therefore, the feed cable from the RF source has to pass through the interior region (between the radiating element and the ground plane) of the PIFA. Such a prior art-feeding scheme of the PIFA will prove to be tedious and cumbersome in the final integration process. An alternative scheme of a PIFA design that circumvents such a tedious feed assembly is always desirable. From the structural and fabrication point of view, an avoidance of a feed cable extending through the interior region of the PIFA is preferred. This invention described hereinafter provides an encapsulated PIFA module in which the feed assembly is confined to the exterior of the module and hence overcomes the existing shortcomings in the final integration process of the prior art.
Keeping in pace with the rapid progress in mobile cellular communication technology, the future design of the cellular handset shall have the provision of more than one antenna to fulfill the additional requirement of BlueTooth (BT) applications. The placement of the additional internal antenna should be accomplished without necessitating any change in the overall size of the handset. The consideration of mutual coupling often warrants the placement of the cellular and BT antennas at different locations on the device chassis with a very small volume earmarked for the BT antenna. In cellular communication applications, multiple antennas may be required to utilize the phone chassis as a common ground plane. In such an application., the internal BT antenna will be an integral part of device chassis. Therefore such an additional internal antenna (for BT applications) such as a PIFA should have the desirable feature of simplified adaptability to the device chassis. A design of such an internal PIFA as a separate module with surface mountable features will be of great importance to facilitate a much simplified integration process.
A compact, lightweight, single band PIFA has been designed in an encapsulated modular form. The present invention emphasizes the feed assembly of the PIFA confined only to the exterior of the module. In the instant invention, one of the external leads of the encapsulated PIFA module facilitates the connection of the feed point of the PIFA to the RF source point of the radio device. The localized ground plane of the PIFA and the ground potential of the chassis of the radio device are connected by the other external leads.